Spacecraft have used several sources of power to operate the payload since the beginning of the space program. In early spacecraft, which were small and had limited capability, batteries met modest power needs. As the complexity and the power needs of the payload grew, primary power was supplied by solar cells. Though solar cells are able to convert sunlight directly into electricity, they have major drawbacks. They have a low efficiency and therefore huge arrays are needed. Further, the power system still needs rechargeable batteries for periods when the spacecraft is in the shadow of the earth. Finally, solar arrays are impractical for distances greater than Mars. The insolation (light energy from the sun per square meter) drops off as the square of the distance from the sun. At Mars, solar power is less than half that received on earth. At Jupiter, it is reduced by a factor of 27, and Saturn, by a factor of 100.
For large power requirements, such as for the Apollo mission, fuel cells were employed. Fuel cells produce power by electrochemically producing the products of combustion. The chemical energy supplied by fuel cells also limits the distance spacecraft can travel because of limitations on the mass of chemicals that can be carried.
When the space programs' goals expanded beyond what chemical power and solar power could provide, NASA requested the Atomic Energy Commission (now the Department of Energy) to develop nuclear power sources. It was decided that the most reliable system would be to use the heat generated by radioactive decay to power thermoelectric devices.
Thermoelectric technology for power generation began in earnest in the late 1950's and a number of semiconducting materials were developed in the Space Nuclear Auxiliary Power (SNAP) program conducted by the AEC. Successful radioisotope thermoelectric generators (RTG) were deployed in the early 1960's to power navigation and weather satellites. The Viking program landed two RTG-containing spacecraft on the Martian surface. The first deep space probe, Pioneer, is powered by RTGs and is expected to operate with them as the only source of power for more than twenty-five years.
Thermoelectic devices are based on the principle discovered by Thomas Seebeck in 1822: when two different electrical conductors are connected in a closed circuit, they will produce electricity if one junction is kept at a higher temperature than the other. This is the phenomenon that allows thermocouples to measure temperature.
The material used for the thermoelectric elements of the RTG is typically a semiconductor material made of a silicon-germanium (SiGe) solution doped to improve its conductivity. This semiconductor material is fabricated into a rod-like thermoelectric device called a unicouple. It generates electricity when one end is hot and the other end is cold. The greater the temperature difference, the more power that is generated. The temperature difference is maintained by insulating the heat source and embedding the unicouples in the insulation so that one end projects from the inner surface of the insulation, adjacent to the heat source. The other end projects from the outer surface of the insulation and is kept cold by radiation into space. Appropriate circuitry collects and distributes the electricity generated.
As spacecraft and rocket launcher technology has grown, the need for higher RTG power output, lower weight, and longer operating times has also grown. One means of increasing power output is to increase operating temperature. However, high operating temperatures require the sealing of the thermoelectric elements to prevent their rapid degradation. Even with sealing, degradation is still a problem. It has been desirable to keep temperatures as high as possible once power is needed and as low as possible before power is needed.
The RTG is subject to two significant power degradation mechanisms. One is radioactive decay of the plutonium isotope that has a half-life of 88 years. This inevitable power loss is about 1% per year. The other mechanism is degradation of the unicouples. The electric power loss due to the unicouple degradation is also about 1% per year.
Before an RTG is put into use, it is stored with a cooling gas. The cooling gas is usually argon. The cooling gas provides conductive and convective heat transfer from the insulation and thermoelectric elements to the outer shell. The cooling seems to be sufficient to eliminate degradation of the thermoelectric elements because the only loss in power is that predicted from radioactive decay alone. With the argon cooling gas, the hot end of the thermoelectric element is about 725.degree. C.
When an RTG is used in space and operating, the insulation is in a vacuum and heat transfer is by radiation to the outer shell and from the outer shell to space. The vacuum conditions begin when conditions in space activate a pressure relief device. The vacuum conditions are then used in the RTG throughout the mission.
At normal vacuum operating temperatures of 1000.degree. C., there are two significant degradation processes that appear to be taking place. One, the unicouples gradually loose their efficiency through some kind of internal degradation. This is thought to be because of migration and precipitation of dopants. Two, the electrical resistance of the unicouple-insulation layer of the RTG drops because of vaporization and deposition of the semi-conducting materials on the surfaces of the insulation. This causes electrical shorting, which decreases overall efficiency of the RTG.
At operating temperatures that are used in space, these two degradation mechanisms account for about 1% power lost per year. However, at the lower storage temperatures of 725.degree. C., neither unicouple efficiency nor electrical resistance changes. Operating in a vacuum and an inner temperature at about 1000.degree. C. balances degradation and efficiency to permit the optimum power output for missions that take 10 years or more to accomplish. Temperatures of 1100.degree. C. produce 25% more power initially, but after 5 years, would produce less power than a unicouple operating at 1000.degree. C. for 10 years.
As the distance to be traveled has continued to increase, various technologies have been used to increase the range of spacecraft. The Galileo spacecraft was the first to use "gravity assists" from planets. In a gravity assist, the spacecraft flies close enough to a planet to accelerate relative to the Sun. In the case of Galileo, it was necessary to first fly by Venus and then return to Earth twice to gain enough velocity to reach Jupiter.
The gravity assist allows more payload for a given launch. But with the gravity assist, the length of time required to accomplish a given mission has increased greatly, as have the periods for which full power is not needed. During the coast period, much of the instrumentation is quiescent and draws no power. Because there is no means to cool the RTG in space, deterioration continues. Thus, if a mission is going to take more than 10 years, it would be desirable to control the operating temperature so that it could be elevated only when greater power was needed.